1. Field of the Invention
The present invention relates to high speed input/output buses, and more particularly to high performance buses optimized for internetworking applications.
2. Description of Related Art
Input/output buses (I/O buses) provide the backbone for communication of data among data processing modules in a data processing system. The bandwidth of the bus is often a determining factor in the performance of the overall system. In large scale communication networks, internetworking devices often couple a plurality of high speed networks to a common controller. The amount of data transferred on the network can be very large. Thus, a very high bandwidth I/O bus is necessary for such systems.
One critical component in the bandwidth of the bus is known as bus latency; that is, the amount of time between a request for access to the bus, and completion of a transfer across the bus. When there are a number of users of the bus, arbitration for access to the bus is often a large component of the latency. Thus, many high speed buses attempt to arbitrate for the bus in parallel with data transfers. For instance, the so-called SBUS promoted by Sun Microsystems, Inc., uses a "radial" bus request/bus grant design for arbitration. In this design, every device on the bus which will seek access to the bus as a bus master, must have a dedicated bus request and bus grant line. Also, each slave on the bus, which is able to receive or respond to requests on the bus, must have a dedicated slave select line. All of these lines are coupled to a centralized controller which performs arbitration in parallel with the address and data transfers occurring for the last arbitration winner. The SBUS suffers the disadvantage, however, of requiring a bus having a large number of lines, which drives up the cost of bus interface devices. The SBUS also requires an independent, high pin count arbiter circuit.
It is desirable to provide a bus having an arbitration mechanism which supports high bandwidth, low latency transfers, and operates with a minimum number of lines on the bus.